1. Field of the Invention
The present invention relates to a process for producing hydrogen cyanide by catalytic ammoxidation using a modified Mn—P catalyst.
2. Description of Related Art
Hydrogen cyanide (HCN) is a very important by-product in nylon synthesis. It is used in the manufacture of methyl methacrylate, nitrilotriacetic acid (NTA, a detergent additive), herbicides, in production of methylthiohydroxybutyric acid (an animal food additive), chelating agents, and many other fine and specialty chemicals. It is used also for gold extraction from ores. Annual production of HCN in United States is about 700,000 tons. About 20% of this HCN is a by-product of acrylonitrile (AN) manufacture: more that 70% is made by DuPont's Andrussow process developed in 1930s, while relatively small amount (less then 10%) is produced via Degussa process.
In the Degussa process CH4 and NH3 react directly inside ceramic tubes coated with platinum. Since the reaction between CH4 and NH3 is highly endothermic (equation 1) high reaction temperatures (1200-1300° C.) are required; the necessary heat is provided by combustion of a fuel on the outside of the tubes.NH3+CH4→HCN+3H2 (ΔHr=252 KJ/mol)  (1)
Since this process operates in the absence of air, a number of side reactions are eliminated. Moreover, the off gas is nearly pure H2 (96.2%) after scrubbing of HCN and NH3. Conversion of NH3 to HCN is 80-85% and that of CH4 to HCN is 90%.
In the Andrussow process air is added to combust a small amount of CH4 to provide necessary heat for the reaction. Thus production of HCN by this latter process is ammoxidation reaction:NH3+2CH4+3.5O2→HCN+CO2+5H2O (ΔHr=−474 KJ/mol)  (2)
The Andrussow process operates adiabatically at about 1100° C. Optimal feed composition is determined by a compromise between selectivity for conversion of NH3 to HCN, favored by a high CH4/NH3 ratio at a fixed air/fuel ratio, and overall HCN production rate, favored by a relatively low CH4/NH3 ratio. Residence time is extremely low (less then 1 ms) to prevent decomposition of HCN product. A typical catalyst in the Andrussow process is 90% wt. Pt and 10% wt. Rh wire gauze. Acceptable catalyst life is 60 to 360 days. About 65-70% of ammonia is converted to HCN and about 10% to N2. The product gases, containing about 6-12% HCN, are rapidly quenched in a waste-heat boiler to 350-400° C. to avoid decomposition of the HCN.
HCN is also produced as a by-product of propylene ammoxidation to acrylonitrile as follows:CH2═CHCH3+3NH3+3O2→3HCN+6H2O (ΔHr=−273 kcal/mol)  (3)
The amount of HCN produced as a by-product in propylene ammoxidation is linked to the amount of produced AN. HCN is a valuable product, and at times its demand exceeds the production of HCN in propylene ammoxidation process. When this happens, methanol is fed together with propylene into the reactors, where it reacts with ammonia and oxygen on AN catalyst to produce HCN as follows:CH3OH+NH3+O2→HCN+3H2O (ΔHr=−83 kcal/mol)  (4)However, AN catalysts are optimized for propylene rather than methanol ammoxidation reaction. Besides, introduction of methanol into the propylene ammoxidation feed may reduce the life of the catalyst.
Therefore, there is a need to decouple AN and HCN production, and develop an inexpensive HCN production process. The process would use a catalyst specifically developed to produce HCN from methanol.
There are a number of known catalysts for the ammoxidation of methanol to HCN based on complex metal oxides such as K0.006Bi0.45Fe0.65P0.1MoOx-50% SiO2 that include almost all elements from the periodic table. However, only a few of them have been really tested in the ammoxidation reaction. Some of the known ammoxidation catalysts along with catalytic properties are presented in Table 1.
TABLE 1Methanol ammoxidation over complex oxide catalysts.CatalyticNCatalyst compositionReaction conditionsproperties*1FeaCubSbcModMeeTefQgOx/SiO2T = 350-500° C.X = 96-100%,Me = V, W; Q = Mg, Zn, La, Ce, Al, Cr,O2:NH3:MeOH = 2:1.1:1Y = 79-93%Mn, Co, Ni, Bi, U, Sn;2RrXqTpZsFetSbuPvOxT = 300-500° C.S = 82-95%R = IA   IB, IIC, X = Bi, Te;O2:NH3:MeOH:H2O =Y = 77-87%T = Cr, Co, Cu. Ce. Th, B, Sn;2.1:1.2:1:2.25Z = V, Mo, W;3RrAaBibCecWdVeMofOxT = 300-500° C.Y = 54-72%R = Cr, Sb; A = K, Na, Rb, Cs, Tl, Sm,O2:NH3:MeOH:H2O =Ag, Cu;2.1:1.2:1:2.4FeaSbbQcRdOx;T = 380-470° C.X = 96-99.8%,Q = V, Co, Ni, Cu, Mo, W, Bi;O2:NH3:MeOH = 1.4:1:1Y = 91-94%R = B, P, K, Zn, Te;5FeaSbbPcXdQeRfOx/SiO2;T = 350-500° C.X = 80-100%,X = V, Mo, W; Q = Li, Na, K, Rb. Cs,O2:MeOH = 1-10S = 89-96%Mg, Ca, La, Ce, Ti, Zr, Nb, Ta, Cr, Mn,NH3:MeOH = 0.7-2.5Y = 70-94%Re, Co, Ni, Cu, Ag, Zn, Al, Sn, Pb;R = B, As, Se, Te;6FeaSbbPcVdMoeCufWgXhYiZjO/SiO2T = 380-470° C.X = 98-100%,X = Mg, Zn, La, Ce, Al, Cr, Mn, Co, Ni,O2:MeOH = 1.3-1.5Y = 78-94.5%.Bi, U, Sn; Y = B, Te;NH3:MeOH = 0.7-1.1Z = Li, Na, K, Rb. Cs, Ca, Ba;7FeaSbbPcVdMoeCufWgXhYiZjO/SiO2T = 380-470° C.X = 97-100%X = Mg, Zn, La, Ce, Al, Cr, Mn, Co, Ni,O2:MeOH = 1.3-1.5Y = 80-92%Bi, U, Sn; Y = B, Te; Z = Li, Na, K, Rb.NH3:MeOH = 1-1.3Cs, Ca, Ba;8FeaSbbPcMxOyT = 440-480° C.X = 82-100%M = V, MoO2:NH3:MeOH = 1.6:1:1S = 93.2-94.7%9FeaSbbPcVxCuyOzT = 440° C.Y = 94.5%O2:NH3:MeOH = 1.5:1:1*X - methanol conversion, S - selectivity to HCN, Y - HCN yield.
A large volume of developmental work on methanol ammoxidation was done by Monsanto in the 1980s as a result of which manganese phosphate based catalysts (Mn1.25POx) were developed for HCN production from methanol. Manganese phosphate based catalysts (Mn1.25POx) are very simple in preparation, and they can provide up to 90% HCN yield from methanol. The process can be run under conditions close to those of the current AN process and could be retrofitted into an existing AN production plant.
Therefore, a catalyst specifically designed for the production of HCN with increased yields over current technology would be highly advantageous.